Phylogeny Friday - 30 March 2007

Mammals did not rapidly radiate after the K/T boundary.

That's the punch line of a paper published in this week's issue of Nature. This has been all over the news, including the New York Times twice (#1 and #2). You see, there's this idea that when the dinosaurs (technically, the non-avian dinosaurs) disappeared, mammals quickly filled in the vacated niches. That means there should have been a rapid radiation of mammalian lineages following the dinosaur mass extinction -- marked by the boundary between the Cretaceous and Tertiary periods (known as the K/T boundary).

The new study reveals that the extant mammalian lineages (those that are present today) did not originate immediately after the dinosaurs went extinct. PZ Myers points out that our concept of a mammalian radiation is biased by our attention to charismatic megafauna. The little mammals -- which make up the majority of mammalian diversity -- wouldn't have been as affected by the K/T extinction event as large mammals, so we won't see much of a phylogenetic signature of the radiation of the big guys. Also, the study focused on extant mammals, and would have missed any radiation of extinct lineages.

I want to focus on another point: that of morphological differentiation and lineage diversification. Oh, and there's a phylogeny below the fold.

The authors of the study constructed a phylogeny using DNA sequences from over 4500 mammalian species -- nearly every extant species -- and showed that the major extant mammalian clades (shown in five different colors on the tree below) originated prior to the K/T boundary (shown as a dashed circle). They also show that there is no evidence for increased phylogenetic diversification after the dinosaur extinction by plotting the amount of lineages versus time.

Click to enlarge.

But does this really address the issue of mammals filling the niches vacated by non-avian dinosaurs? Evolutionary lineages are different than morphological diversification. The authors showed that there wasn't any rapid diversification of the sampled lineages. But these are genetic lineages, which do not correlate perfectly with morphological differentiation. It's possible that many of the mammalian lineages were present prior to the K/T boundary, but they didn't morphologically differentiate until after the dinosaur mass extinction.

What we have is evidence against rapid speciation after the dinosaur extinction. We do not have evidence that there wasn't rapid evolution of phenotypes after the extinction (sorry for the double negative). These are two separate hypotheses which must be addressed using different sets of data.

Bininda-Emonds ORP, Cardillo M, Jones KE, MacPhee RDE, Beck RMD, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A. 2007. The delayed rise of present-day mammals. Nature 446: 507-511. doi: 10.1038/nature05634

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You make a good point. The paper is very cool and important, but it also does not reflect the actual fossil record in another way: The molecular phylogeny is based only on living forms. If a number of mammal species arose right after the KT boundary, then later contributed only a little or not at all to the present diversity of mammals (because of subsequent extinction) then this molecular phylogeny is not a test of the post KT event diversification hypothesis.

This may be the case, as the fossil record contradicts the molecular phylogeny. See:

The extant/extinct dinstinction was PZ's, not mine. My point was that genetic diversification is different from morphological diversification.

Well, this layman still thinks you made a good point. Except that this suggests further that perhaps speciation, and genetic and morphological diversification may be three somewhat different things. (With copy number variation and all.)

Still, it would be interesting to see the (extinct) record of more lineages compared. Again, as a layman, if to start with extinctions could be pinned down and possibly (group-wise) explained already that would be an impressive description of some of the background for diversity.

Which brings me to the term of "vacated" niches.

I would naively think a few disappeared niches in an existing ecology could be, contingently, filled. I could think that coarse grained contingency points to categories of niches such as plant eaters and predators - in fact, it would be hard not to.

But if the K-T boundary meant that wholesale animal and plant groups temporarily diminished or disappeared, it is much harder for me to think of contingent niches as just vacated instead of destroyed.

By Torbjörn Larsson (not verified) on 31 Mar 2007 #permalink

disappeared niches

It is not so impressive to discuss terms while misusing them. I didn't mean that the niches disappeared here, I meant that they were just vacated.

By Torbjörn Larsson (not verified) on 31 Mar 2007 #permalink

Except that this suggests further that perhaps speciation, and genetic and morphological diversification may be three somewhat different things. (With copy number variation and all.)

All of this is separate from CNVs -- I'm not sure how they would affect anything.

The genetic divergence between any two taxa predates the speciation time by about 4N generations (N=population size of the ancestral species at the branching point). This should only matter for recent speciation events -- the difference in times will become vanishingly insignificant if the species diverged a long time ago.

And there's no way (without other evidence, such as fossils) to relate molecular divergence dates with morphological differentiation.

All of this is separate from CNVs -- I'm not sure how they would affect anything.

On second thought, I'm not sure either.

I was trying to get to Moran's concept of near neutral variation. (Which this layman doesn't know much about, of course.)

By Torbjörn Larsson (not verified) on 02 Apr 2007 #permalink